近日，瑞士苏黎世联邦理工学院Max W. Schmidt团队研究了碳酸盐岩熔体的钠质性质及方解石与白云石碳酸盐岩的成因。2025年12月22日出版的《地质学》杂志发表了这项成果。

碳酸岩作为岩浆岩，是大多数经济型稀土元素矿床的赋存载体，并关乎地球深部挥发分循环。然而，深成碳酸岩的累积特性与火山碳酸岩喷发后的快速蚀变，阻碍了学界对结晶形成这类岩石的熔体成分进行直接测定。

研究组聚焦于碳酸岩熔体中的钠元素——作为其成因、岩浆演化的关键组分，同时鉴于多数碳酸岩型稀土矿化是由富碱流体形成，钠也对稀土元素的运移至关重要。通过在次火山条件（100–200 MPa，800–1000 °C）下开展实验，研究组测定了钠在磷灰石与碳酸岩熔体间的分配系数（D_Na^{磷灰石/碳酸岩}）。该系数在温度≥800 °C时较为稳定，为0.010 ± 0.003。将所得分配系数应用于天然样品，推算出形成钙质碳酸岩的熔体含8–22 wt% D_Na^{磷灰石/碳酸岩}，而形成白云石质碳酸岩的熔体含26–32 wt% Na₂O。因此，后者比钙质碳酸岩熔体演化程度更高，从而更易于释放富碱流体。

通过建立模型，考虑磷灰石饱和前从钙质碳酸岩熔体中发生的方解石分离结晶作用，表明在900 °C时原始熔体的Na₂O^meltO含量为16–23 wt%，在1100 °C时为9–11 wt%。这一结果与碳酸岩熔体由碱性硅酸盐岩浆经液态不混溶作用形成的观点相符，但与直接源自地幔的成因模式不一致——尤其对于白云石质熔体而言。

附：英文原文

Title: Sodic nature of carbonatite melts and the origin of calcite versus dolomite carbonatites

Author: Simone Marioni, Gino Sartori, Max W. Schmidt

Issue&Volume: 2025-12-22

Abstract: Carbonatites are magmatic rocks that host most economic rare earth element (REE) deposits and bear on the deep Earth volatile cycle. Yet, the cumulative character of plutonic carbonatites and the rapid posteruptive alteration of volcanic carbonatites impede a direct determination of the melt compositions crystallizing these rocks. This study focused on sodium in carbonatite melts, a critical component for their genesis, magmatic evolution, and also for REE mobility, given that most carbonatite-hosted REE mineralizations are formed by alkali-rich fluids. Through experiments at subvolcanic conditions (100200 MPa, 8001000 °C), we determined the partition coefficient of Na between apatite and carbonatite melt, DNaapatite/carbonatite, which is rather uniformly 0.010 ± 0.003 at temperatures ≥800 °C. Applying our DNaapatite/carbonatite value to natural samples yields 822 wt% Na2O for the melts forming calcic carbonatites and 2632 wt% Na2O for melts forming dolomite carbonatites. The latter are hence more evolved than calcic carbonatite melts, increasing their potential to shed alkali-rich fluids. A model accounting for calcite fractionation from calcite carbonatite melts prior to apatite saturation indicates primary Na2Omelt contents of 1623 wt% at 900 °C and 911wt% at 1100 °C, consistent with formation by liquid immiscibility from alkaline silicate magmas, but inconsistent with direct mantle derivation, in particular for dolomitic melts.

DOI: 10.1130/G53916.1

Source: https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G53916.1/723700/Sodic-nature-of-carbonatite-melts-and-the-origin